Face/Off, the action thriller film, tells the story of two sworn enemies who assume one another’s appearance. Unlike their screen counterparts, Nicolas Cage and John Travolta, George Osborne and Ed Balls have not swapped facial characteristics. In many other respects, however, they have grown eerily alike.

Balls/Osborne, The Movie, which reaches a denouement in today’s Spending Review, has chronicled how adversaries with almost nothing in common have developed uncanny similarities. First the shadow chancellor confirmed that he would apply a cap on structural expenditure and stick to overall Tory cuts for 2015/16. Not to be outdone, the Chancellor discovered his inner Keynes and pre-announced the push on infrastructure spending to be detailed by his Lib Dem deputy tomorrow. Having copied Mr Balls’s focus on growth, Mr Osborne hinted that the Tories would also follow his adversary’s lead and cut fuel allowances for richer pensioners.

Cries of “Too far, too fast” (on Mr Balls’s part) and grim tales of Labour profligacy (on Mr Osborne’s) have mutated into what many see as a double bill in the tradition of Ray Alan and Lord Charles, or Rod Hull and Emu. The question yet to be answered in the voters’ minds is which of the antagonists is the ventriloquist and which the dummy.

Late on the night before he delivered the speech recalibrating Labour’s economic approach, Mr Balls sat at his kitchen table and applied his new-found enthusiasm for cuts to his own handiwork by hacking 1,000 words of detail out of his address. If that last-minute revision betrayed anxiety, no one should be surprised. Mr Balls’s feat of simultaneously attacking and emulating Mr Osborne was never going to be easy.

Having decided that the state of the economy demanded a shift, he and Ed Miliband agreed they must act now to avoid the trap sprung by a Chancellor who intended to set out his own spending plans, and then ask what Labour would do instead. Mr Balls had told allies that, in switching course, he “wouldn’t do it by halves” and that the message he wanted voters – and his colleagues – to hear was “iron discipline”. That analysis does not console thoughtful critics, such as the Compass think tank and Peter Hain. As one leading Leftist says: “We didn’t sign up with Ed Miliband for this.” Meanwhile, those on the party’s Right are wondering why Mr Balls waited for so long.

If Labour is taking a risk by being tougher than the Chancellor may be today, then so is Mr Osborne. His Spending Review has been impeccably choreographed, with ministerial mutinies, capitulations and sops to Nick Clegg, culminating in the £11.5 billion cutbacks he desired. But Mr Osborne is failing, not only on Mr Balls’s terms but on his own. With promised economic growth still absent and deficit reduction far off course, the Government is now looking to borrow £245 billion more than it planned up to 2015.

The Balls/Osborne fight is the clash of two strategic titans, both hampered by the volatility of forces outside their control. In this uneasy climate, Ed Miliband is confident that the Eds’ reorientation has sown confusion among Tories who don’t know whether to attack them as the “same old Labour” or to crow that they have seen the light.

Mr Balls, who believes that Mr Osborne is running out of options, has also told friends that Mark Carney, the incoming governor of the Bank of England, cannot be “George Osborne’s salvation”. The shadow chancellor, who knows and admires Mr Carney, may be right. It is possible, too, to think that if the economy finally starts to grow slowly, as Mr Balls expects, then the new governor may raise interest rates far sooner than is comfortable for the Government.

But Mr Osborne’s problems are also Labour’s woes. Today’s cuts are merely a warm-up, with the Institute for Fiscal Studies projecting that Mr Osborne’s future plans will mean a further £23 billion reduction in 2017/18. What could Labour (or the Tories) do in the face of cutbacks that would dismantle Britain’s already crumbling institutions?

The ills afflicting the NHS, for example, go far beyond furores over regulation and ethos. Mr Balls expects a funding crisis even before the next election, and Labour so far lacks an agreed plan on how fully to pay for the consolidated health and social care service it wants. By contrast, there are bold savings to be made in other departments, such as defence, if Mr Miliband – who is still considering whether to defer or scale back on Trident renewal – dares to do so.

Although excessive caution may bedevil the Opposition, the leadership has shown it is prepared to play the long game that this economic perma-crash requires. Mr Balls’s wisdom in addressing wealthier pensioners has been highlighted by Mr Osborne’s seeming wish to follow suit. Although he was punished for a lack of clarity, Mr Balls was right, too, to flag up pensions as an area in need of future reform. Expect much more to come on raising retirement ages and, no doubt, on pension tax relief for higher earners.

Mr Miliband, meanwhile, is branding his new line of attack as “discipline and difference” – themes that are being developed in a partly written conference speech whose working title could be: What I Shall Do For Britain. Behind the sloganeering is a programme to spell out who should bear the biggest burden, and to tax and cut accordingly while sticking to 2015 spending limits.

Secondly, he hopes to paint much more clearly a (still hazy) economic landscape featuring better technical education, a living wage, regional banks and that still-uncracked old chestnut, predistribution. Finally, he will “soon be setting out” his infrastructure plans for housing, green energy, schools and transport, as well as making the case for the increased capital spending he rightly deems vital.

Labour, conscious that Mr Osborne will claim that his own infrastructure plans are already costed, will be watching out this week for the pre-electoral manoeuvring at which Mr Osborne excels. “George talks about schemes for named marginal constituencies,” says one Labour luminary. “Whereas Gordon always specified X thousand miles of rail track.”

Mr Miliband, needless to say, is not channelling Gordon Brown. Instead he is modelling himself on Clement Attlee, the Labour alchemist who rebuilt the welfare state and ran a budget surplus in a time of penury. “It’s about doing more without getting out the chequebook,” says one strategist. Beguiling as that template may be, Attlee’s universe was very different. As the social historian, David Kynaston, wrote of 1945 in Austerity Britain: “No supermarkets, no motorways, no tea bags, no duvets, no Pill, no trainers, no hoodies, no Starbucks. Four Indian restaurants.” Almost the only unrationed commodity was the hope, now so singularly lacking, for a better world.

Mr Miliband is said to believe that taking his fight to the enemy has put him “in a much better place”. The question is whether the electorate will feel equally transported. The two Eds must, in the next few weeks, show they can carry off their double act as Attlee and his chancellor, Sir Stafford Cripps. Should they fail, then voters may instead perceive them as Cameron and Osborne in disguise.